1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to an image sensor having a color filter with a slope interface and a method for fabricating the same in which the area of a condensing lens is maximized and a phase signal of uncondensed light is absorbed so as not to generate photoelectric conversion, thereby obtaining better resolution and photosensitivity.
2. Discussion of the Related Art
Generally, in fabricating an image sensor, the resolution is determined by the number of photodiodes existing in an image plane that receives images. Therefore, there is a trend in the industry toward a high number of pixels and miniaturization of the unit pixel in the image sensor. In condensing external images to the image plane, the size of the unit pixel is reduced and a light-receiving portion is reduced, thereby reducing photosensitivity. To enhance photosensitivity, a condensing lens can be used.
The condensing lens is formed below or on color filter layers. Since condensing efficiency of incident light depends on a sectional area of the condensing lens, the area of the condensing lens is maximized to condense more light Uncondensed light is reduced to reduce a phase signal, thereby obtaining images at a high resolution.
As described above, with the miniaturization and a multi-pixel structure of the image sensor, more pixels per unit area can be formed. With the decrease in pixel size, the sizes of color filter layers and a microlens layer formed in an on-chip mode also become smaller. As the size of unit pixel is reduced, a photodiode area that receives light is also reduced, thereby reducing photosensitivity. To compensate for reduced photosensitivity, an inner lens can be additionally formed. In this case, the inner lens induces the incident light to adapt to a variation of a the condensing angle due to F-number and compensates for stray light at an increased distance from the photodiode area. Alternatively, to compensate for reduced photosensitivity, the size of the microlens layer is maximized.
In addition, to compensate for any reduction in photosensitivity, the photodiode area should receive as much light as possible. To increase the amount of light received by the photodiode area, an opening can be increased and a condensing microlens can be formed. The opening is formed by a metal layer that serves as wiring and for light-shielding. In condensing the incident light entering the light-shielding layer through the unit pixel to the opening after refracting the light through the condensing lens, the adhesion between the condensing lenses is varied depending on the size of the condensing lens. For this reason, image uniformity is deteriorated. The condensing lens can be affected by the color filter layers in such that information between adjacent color filter layers is mixed, thereby deteriorating color reproduction and contrast. Moreover, it is difficult to form a precise pattern during alignment exposure due to poor optical resolution caused by the mixture of pigment when the color filter layers are formed. A planarization layer is additionally required because of the overlap or gap between the color filter layers.
FIGS. 1A to 1E are sectional views illustrating processes for fabricating a conventional image sensor.
Referring to FIG. 1A, photodiodes 12 are formed in a substrate 10, and metal layers 14 are formed over the substrate 10.
Referring to FIG. 1B, a planarization layer 20 is formed over the entire surface of the substrate, including the metal layers 14, to minimize any overlap or gap between the subsequently formed color filter patterns.
Referring to FIGS. 1C to 1E, the color filter patterns are formed on the planarization layer 20 by a photolithographic process to perform color filtering. In other words, the photolithographic process is performed three times to sequentially form the R/G/B color filter patterns.
FIGS. 2A to 2D are sectional views illustrating processes for forming the color filter patterns for conventional image sensor in more detail.
FIG. 2A illustrates a profile of a first color filter pattern 60 in a conventional image sensor. FIG. 2B illustrates a profile of second color filter layer 70 formed on the passivation layer 20, including the first color filter pattern 60. FIG. 2C illustrates a profile between the respective color filter patterns in the conventional image sensor. Referring to FIG. 2C, an overlap 80 is formed on the second filter pattern 70a adjacent to the first color filter pattern 60. FIG. 2D is a sectional view illustrating an overcoating layer 90 formed on the color filter patterns 60, 70 in the conventional image sensor.
When color filter patterns are formed by sequentially performing a photolithographic process three times, the profile of the first color filter pattern can play a role in determining the characteristics of the image sensor. Generally, a sidewall of a color filter pattern is formed in a negative pattern at an angle of 90° or greater. A step difference is formed at the edge of the first color filter pattern due to the surface topology when the second color filter pattern is formed. The second color filter pattern becomes thicker at the overlapped portion with the first color filter pattern due to the step difference. Moreover, the pattern profile of FIG. 2C is obtained by various process factors such as mask design during alignment exposure, wafer alignment, development, and other factors.